1. Field of the Invention
The present invention relates to a method for defining an error compensation for the difference between a desired quantity of liquid and the quantity of liquid actually drawn into a disposable pipette tip mounted on a plunger-operated pipette, including means for adjusting the stroke of a plunger and means for indicating the desired quantity of liquid, wherein the relation of the stroke of the plunger with respect to the quantity of liquid indicated is defined by a plunger-correcting factor, with the pipette tip comprising an opening communicating with an operating portion to be filled with the desired quantity of liquid, and a mounting opening for releasably connecting to the plunger-operated pipette, and with a dead volume being defined between the plunger and the opening communicating with the operating portion.
2. Description of the Prior Art
German Patent 25 26 296 discloses a pipette system of the type referred to in which the piston-operated pipette includes a tapered portion for mounting a disposable tip. The inner space of the tip between the tip opening and the mounting opening comprises a first portion which is strongly tapered and an adjacent second portion which is slightly tapered. When such a tip is inserted in a deep and particularly slim container, there is the drawback that the pipette on which the tip is mounted may contact the inner wall of the container resulting in a contamination. Basically, this is avoided when using a tip disclosed in European 0 182 943, according to which the tip opening extends into a long cylindrical nose portion followed by a strong tapered portion and a mounting opening.
With the known tips, all internal spaces having varying cross sectional areas are used to be filled with liquid depending on the liquid volume drawn into the tip. Accordingly, liquid will be drawn into the strongly tapered or, respectively, cylindrical initial portion when a small piston stroke is executed, while liquid will be drawn further into the slightly tapered portion or any other adjacent portion which may be tapered or cylindrical when the piston stroke is increased. All these pipette systems suffer from the drawback that the quantity of liquid drawn into the tip by operating an adjustable plunger-actuated pipette is substantially different from the desired quantity of liquid to which the pipette is set, and which quantity is indicated to the user on a scale or the like. In particular the error between the desired and the actual quantity of liquid may vary within the range of liquid volumes to which the plunger may be set. As the plunger stroke is set by rotating a spindle to adjust a stop, the pitch of the spindle is selected such that the error between the desired quantity of liquid and the actually taken-up quantity of liquid is tolerable across the full adjusting range. The errors resulting have to be accepted, but could be determined by taking measurements afterwards.
Besides accepting these errors of unknown magnitude occurring within the full adjusting range, there is the further drawback that exchanging one tip against another tip results in further errors as the plunger-operated pipette and the tip are selected to be in mutual relation. For example, when a tip of a first type is changed against a tip of a second type, the pitch of the spindle once selected will result in unpredictable errors.
American Patent 5 024 109 discloses a method and an apparatus for performing a correction in a piston-operated pipette. The technique involves determining the height of fluid in the tip for a desired fluid volume, determining the change in dead volume resulting from having fluid of such height in the tip and determining the piston displacement required to achieve a volume equal to the sum of the desired volume and the determined change in dead volume. The height of fluid in the tip is determined from the Volume multiplied by values which are constant for a given tip size and geometry. The change in dead volume varies as a function of the desired fluid height times a constant for a given fluid, pipette system and tip. The technique makes use of an electromechanical pipette and a control unit including a processor which is coupled thereto. Before use, constants for tips, system and fluid have to be stored into memory of the control unit. This technique is expensive.
Accordingly, it is an object of the present invention to improve a pipette system of the type referred to. Particularly, the amount of liquid to be taken up between the minimum and the maximum plunger stroke shall be determined with lower expense and a high accuracy. Still further, it shall be possible to use a plurality of different plunger-operated pipettes and tips where the errors occurring are still acceptable.